What if it is the water coming up off the tires and smacking the inner fender wells? When I was washing it I tried to spray different things and it didn't seem to do anuthanyat all until I got to the the fenders. It would make sense of why it stops when you stop.Drove yesterday in the rain and my wife says, "Why does it sound like rain hitting a tin roof in here?" Whomever said it earlier, was spot on. I guess it could be the plastic, def. the windshield area.